1. Field of the Invention
The invention relates to a process for the preparation of chromium metal powders having a low oxygen content and an average particle size not greater than 20 .mu.m by metallothermal reduction of a mixture of chromium oxide and calcium oxide with calcium metal in a reactor to which access of oxygen is prevented, at temperatures of 1,000 to 1,250.degree. C. and an initial pressure of not greater than 10.sup.-3 bar at room temperature.
2. Description of the Prior Art
It has long been known that metal oxides, such as, for example, the oxides of uranium, zirconium, vanadium, titanium or chromium, can be reduced with alkaline earth metals, especially with calcium metal. For example, German patent No. 441,640 describes a process in which the oxide is heated with an alkaline earth metal, e.g., calcium, and a halide of the same or a different alkaline earth metal, e.g., calcium chloride or barium chloride, or an alkaline metal, for example, potassium chloride. However, the metal powder formed is relatively coarse grained and still contains relatively large amounts of oxygen. This oxygen content is attributable to the fact that unreduced metal oxide is enclosed in the coarse particles of metal powder.
German Auslegeschrift No. 10 30 033 relates to a process of the above type which is characterized by the fact that the reducing metal and the oxide to be reduced are introduced into a melt of alkali and alkaline earth halides, which melt has been dried by prolonged heating. This procedure does achieve a reduction in the unreduced metal oxide content. However, at best, a coarse-grained metal powder or metal regulus rather than a finely particulate metal powder is obtained.
German patent No. 935,456 discloses a process for the preparation of alloy powders by reducing metal compounds or mixtures of metal oxides with calcium. The addition of indifferent oxides to the reducing mixture is recommended in this patent. This addition is intended to prevent the melting of the alloy due to the strong evolution of heat during the reduction process or to prevent that the powder formed is very coarse. It has turned out, however, that although a chromium metal powder lower in oxygen can be obtained, this powder is still relatively coarse grained and has a particle size in excess of 100 .mu.m.
Finally, a process for preparing titanium-based sinterable alloy powders by the calciothermal reduction of a mixture of the metal oxides of the metals forming the alloy is disclosed in Germany patent No. 30 17 782. This process is characterized by the following steps:
(a) titanium oxide is mixed with the other alloying components in amounts, based on the metals, corresponding to the desired alloy, alkaline earth oxide or alkaline earth carbonate is added in a 1 :1 to 6 :1 molar ratio of metal oxides, which are to be reduced, to alkaline earth oxide or carbonate, the mixture is homogenized, calcined for 6 to 18 hours at temperatures of 1,000 to 1,300.degree. C., cooled and comminuted to a particulate size not greater than 1 mm;
(b) small pieces of calcium, in an amount equivalent to 1:1.2 to 1:2.0 of the oxygen content of the oxides to be reduced, as well as a booster, in a 1:0.01 to 1:0.2 molar ratio of oxide to booster, are added; this reaction formulation is mixed, the mixture is compressed into green compacts and filled into a reaction crucible which is then closed;
(c) the reaction crucible is transferred to a heated reaction furnace which can be evacuated, the reaction crucible is evacuated to an initial pressure of 1 .times.10.sup.-4 to 1.times.10.sup.-6 bar and is heated for a period of 2 to 8 hours to a temperature of 1,000 to 1,300.degree. C. and then cooled, whereupon the reaction product is removed from the reaction crucible and comminuted to a particle size not greater than 2 mm.
It is an important characteristic of the process of German patent No. 30 17 782 that instead of being reduced calciothermally, the mixtures of the metal oxides to be reduced are first calcined to a compound oxide system, the number of phases of which is smaller than the sum of the starting components. By means of this calcining process, it is possible to produce an alloy powder, the particles of which all have the same composition and structure.